1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a method of fabricating an IPS mode LCD and a method of forming an alignment layer in the IPS mode LCD, capable of improving the image quality of the LCD.
2. Discussion of the Related Art
Generally, a cathode ray tube (CRT) has been most widely used among display devices to display image data on a screen. The CRT is inconvenient because of its large volume and heavy weight compared with its display area.
With the development of electronic industries, the display devices that were previously limited to a television (TV) and a monitor are being used, for example, in personal computers, notebook computers, wireless terminals, vehicle instrument panels, and electronic display boards. Also, due to the development of information communication technology, it is necessary to transmit a large capacity of image information. Therefore, the importance on a next generation display device capable of processing and displaying the large capacity image information increases gradually.
Such next generation display devices are required to be light weight, have a slim profile, high brightness, a large-sized screen, low power consumption, and a low price. Among such next generation display devices, a liquid crystal display device (LCD) draws attraction.
The LCD exhibits a better resolution than other flat displays and has a rapid response time compared to that of the CRT in implementing a moving picture.
As one of the LCDs that are widely used at the present time, there is a twisted nematic (TN) mode LCD. In the TN mode LCD, after electrodes are respectively formed on two substrates and liquid crystal directors are aligned such that they are twisted by 90°, a driving voltage is applied to the electrodes to drive the liquid crystal directors.
However, the TN mode LCD has a serious drawback of a narrow viewing angle.
Recently, LCDs employing a new mode are being actively researched to solve the narrow viewing angle of TN mode LCDs. Examples of the new mode LCDs, include an in-plane switching (IPS) mode LCD, and an optically compensated birefringence (OCB) mode LCD.
The IPS mode LCD generates a horizontal electric field so as to drive the liquid crystal molecules in a horizontal direction with respect to the substrates by forming two electrodes on a single substrate and applying a voltage between the two electrodes. In other words, the major axis of the liquid crystal molecule does not stand up with respect to the substrates but rotates horizontally.
The IPS mode LCD has a small variation in the birefringence of liquid crystal according to a visual direction and thus has an excellent viewing angle characteristic compared to the TN mode LCD.
A related art IPS mode LCD will be described in detail with reference to the accompanying drawings.
FIG. 1 is a sectional view of a related art EPS mode LCD.
The related art IPS mode LCD includes a first substrate 118, a second substrate 119, and a liquid crystal layer 130 interposed between the first substrate 118 and the second substrate 119. The first substrate 118 and the second substrate 119 are attached opposite to each other. First, a metal is deposited on the first substrate 118 and patterned to form a plurality of gate lines and a gate electrode 109. The gate electrode 109 is branched from a gate line and disposed in a thin film transistor (TFT).
Next, a gate insulating layer 120 is formed on the substrate including the gate electrode 109. A semiconductor layer 115 having an active layer 115a and an ohmic contact layer 115b is formed on the gate insulating layer 120.
A data line 110 is formed on the gate insulating layer 120. The data line 110 and the gate line form a matrix configuration.
At this point, in forming the data line 110, source and drain electrodes 116 and 117 of the TFT are formed simultaneously.
A common line and a common electrode 113 are formed in parallel to the gate line.
A passivation layer 128 is formed on a resultant structure.
Thereafter, a data electrode 114 is formed such that it is electrically connected to the drain electrode 117 and is in parallel to the data line 110.
A first alignment layer 129 is formed on the resultant structure.
A black matrix 121 is formed on the second substrate 119 to prevent light leakage. A color filter layer 122 including red, green and blue color filter patterns is formed in an opening of the black matrix 121.
Next, on overcoat layer 123 is formed on the color filter layer to planarized the surface of the color filter layer and protect the color filter layer 122.
Then, a second alignment layer 126 is formed on the overcoat layer 123.
FIGS. 2A and 2B are sectional views of the related art IPS mode LCD when in an off-state and an on-state, respectively.
FIG. 2A illustrates the off-state of the IPS mode LCD. Because a horizontal electric field is not applied, there is no motion in the liquid crystal layer 211.
FIG. 2B illustrates an arrangement of the liquid crystal when a predetermined voltage is applied (that is, in the on-state). There is no phase change of the liquid crystal 211a disposed at a position corresponding to the common electrode 217 and the pixel electrode 230. However, due to the horizontal electric field K generated when a predetermined voltage is applied between the common electrode 217 and the pixel electrode 230, the liquid crystal 211b disposed between the common electrode 217 and the pixel electrode 230 is arranged in a direction identical to the horizontal electric field K.
That is, the IPS mode LCD has a wide viewing angle because the liquid crystal moves along the horizontal electric field.
FIG. 3 is a flowchart illustrating a method of fabricating a related art IPS mode LCD.
In operation S100, top and bottom substrates having the structure of FIG. 1 are manufactured.
In operation S110, a cleaning process is performed to remove foreign particles from the substrates. In operation S120, polyimide (PI) (a solution for an alignment layer) is printed on the substrates using an alignment printing apparatus.
In operation S130, a solvent contained in the solution is dried by applying a high-temperature heat to the solution for the alignment layer and a hardening process is then performed.
In operation S140, a surface of the hardened alignment layer is rubbed in one direction using a rubbing device, thereby forming grooves.
In operation S150, an adhesive seal pattern is formed at an edge of the top substrate, except a liquid crystal injection hole. Then, spacers are dispersed on the bottom substrate.
In operation S160, the two substrates are attached opposite to each other with an accuracy of several μm so as to prevent light leakage.
In operation S170, the attached substrates are cut into unit cells. This cutting process includes a scribing process for forming lines on the top and bottom substrates and a breaking process for dividing the scribed substrates into unit cells by applying an impact thereon.
In operation S180, liquid crystal is injected through an injection hole into a gap between the two substrates that are cut into cells, and the injection hole is then sealed to complete the fabrication of the LCD.
Here, the physical characteristic of the liquid crystal is changed by a molecular arrangement state thereof, and thus there occurs a difference in the response to an external force such as an electric field.
Due to the characteristic of the liquid crystal molecule, a control technique for an arrangement state of the liquid crystal molecule is essential for the study on the physical property of the liquid crystal and the construction of the LCD.
Specifically, a rubbing process for uniformly aligning liquid crystal molecules in one direction is essential for a normal driving of the LCD and a uniform display characteristic thereof.
The alignment layer forming process for determining an initial alignment direction of the liquid crystal molecules will now be described in detail.
The forming of the alignment layer includes a process of depositing a high polymer thin layer and a process of aligning an alignment layer in one direction.
The alignment layer is formed of a polyimide-based organic material and is aligned using a rubbing process.
According to the rubbing process, the polyimide-based organic material is deposited on a substrate and a solvent thereof is volatilized at about 60-80° C. Thereafter, the deposited material is hardened at about 80-200° C. to form an alignment layer. The alignment layer is rubbed in one direction using a rubbing cloth such as velvet to thereby form an alignment direction thereof.
This rubbing process enables an easy and stable alignment process and is thus suitable for mass production of the LCD.
However, the rubbing process may cause a defect when the alignment layer is rubbed using a roller wrapped with a poor rubbing cloth.
That is, since the rubbing process is performed through a direct contact between the rubbing cloth and the alignment layer, liquid crystal cells may be contaminated due to particles on the rubbing cloth. Also, TFTs may be damaged due to an electrostatic discharge, an additional cleaning process may be required after the rubbing process, and the liquid crystal molecules may be non-uniformly aligned in a wide-screen LCD. Consequently, the production yield of the LCD will be degraded.
FIGS. 4A and 4B are respectively a sectional view and a plan view illustrating the alignment state of the liquid crystal in a stepped portion when electrode patterns such as the pixel electrode and the common electrode are formed in the pixel region in the related art IPS mode LCD.
In order to improve the viewing angle, a super IPS (S-IPS) mode LCD may be applied to the existing IPS mode LCD. Also, the IPS mode LCD is manufactured using 3-4 masks so as to reduce the number of manufacturing processes. However, the step difference of the substrate is increased. Therefore, the alignment defect increasingly occurs during the rubbing process.
In FIGS. 4A and 4B, because the alignment layer 351 is formed on the pixel electrode 330 patterned on the bottom substrate, a step difference occurs in an edge region of the pixel electrode 330.
The color filter layer 360 and the alignment layer 352 are formed on the top substrate facing the bottom substrate, and the liquid crystal layer 390 is formed between the top and bottom substrates.
Due to the step difference occurring in the edge of the electrode patterns within the pixel region, the alignment is not well achieved. Thus, causing problems in driving the liquid crystal.
If the liquid crystal is in a normally-black mode, a black color is displayed when no voltage is applied.
However, light leakage occurs in regions A of FIGS. 4A and 4B in the off-state when no gate voltage is applied.
That is, when no voltage is applied, the liquid crystal molecules must be aligned in the same direction as the rubbing direction of the alignment layers 351 and 352.
However, the stepped edge portion of the electrode 330 causes a non-uniform liquid layer 391 having an alignment direction different than the rubbing direction and also causes the liquid crystal of a uniform liquid layer 392 to have an alignment direction, different than the rubbing direction.
The non-uniform liquid crystal causes phase retardation of light. The phase retardation causes a linearly-polarized light to change into an elliptically-polarized light. The elliptically-polarized light causes phase retardation in the uniform liquid crystal layer formed near the color filter layer, resulting in a great phase retardation.
Consequently, when no voltage is applied in a normally-black mode, light of the backlight assembly passes through the region A. This causes light leakage in a black display state and a decrease in a contrast ratio, thereby making it difficult to implement a high image quality.
In order to improve the viewing angle, an S-IPS mode LCD is applied. Also, an IPS mode LCD is manufactured using 3-4 masks to reduce the number of manufacturing processes. In these IPS mode LCDs, the step difference of the substrate is increased to cause an increased alignment defect.
Accordingly, there is required a display device and method for preventing the degradation of image quality due to the stepped edge portion, such as an increase in a black brightness and a contrast ratio.